The invention relates to video signal processing systems including systems for processing television video signals for editing purposes and graphic systems for creating, retouching or editing pictures intended to be displayed, televised or printed. The term video signal processing is intended also to include the transmission of video signals.
Video signal processing systems are often designed to process video signals in digital form. For example, in a color processing system an individual picture point signal, or pixel, may comprise three eight bit binary digital signals representing, say, R.G.B, or Y.U.V, or C.Y.M, components of the pixel. The use of digital video signals facilitates processing and reduces degradation of the information due to noise during the processing. However it limits the number of luminance or color values which can be represented by the processed signals, depending on the number of bits in the digital signals, and in some cases can produce noticable effects which are undesired. This may happen for example if a gradual change in the luminance or color value is represented by the digital signals. The digital signals may step from one value to an adjacent value along a line (not necessarily horizontal) in the picture, and remain constant at the new value over a band in the picture until the next step in value occurs. The change in luminance or color at a step may be small, but nevertheless can be disturbingly perceptible especially if the picture is printed or shown as a still picture on a monitor screen. The steps may have the appearance of contours.
Such effects are particularly undesirable, but unfortunately likely to occur when it is desired to produce a vignetting effect or a so called color wedge on a printed or displayed picture. To produce such an effect, the luminance or color value of the picture is required to change gradually from an initial luminance or color value to an end value over a predetermined distance on the picture. The effect could be produced by specifying two end colors for the wedge, for example dark blue and light blue, and the positions of the beginning and end of the wedge. Signals are generated to produce a line of dark blue at the beginning of the wedge and light blue at the end of the wedge, and lines in between are generated in gradually changing shades of blue with the aid of a computer. Each of the color component signals may be represented by eight bits, allowing only a limited number of shades, with the result that there are distinct changes in color between lines, which can result in a stripy display. The problem can be aggravated in printing, as usually fewer color or luminance steps are available compared with television.
It has been proposed to resolve this problem by adding noise to the digital signals, but the noise tends to cause an unacceptable general deterioration or the picture before the contours or steps are obscured. Furthermore an article entitled "Shaded Computer Graphics in the Entertainment industry" by Franklin C. Crow, University of Texas, published in Computer, March 1978 refers to a 2-D animation system designed at NYIT including an interactive shaded drawing system. An Airbrush can be simulated by scattering random dots of proper color on the image in a probabilistic distribution around a position sent from a data tablet. The problem discussed in the proceeding paragraphs is not touched upon on this article.
U.S. Pat. Nos. 3,739,082, 4,275,411, 4,460,924, 4,568,966 and 4,652,905 describe various systems for dither-quantized signalling for color television. In these systems, the video components are coarsly quantized before transmission to a receiver. To prevent loss of perceptible picture information in consequence of the course quantisation of the signal amplitude and to minimise visible artefacts, suitable ordered dither is first added to the signal before it is quantized. A black and white picture, for example, transmitted by dithered and coarsely quantized signalling is reproduced having a stippled structure such that a shade of gray intermediately between quantum signal levels is approximated by means of a mixture of dots, some lighter and some darker than the required shade. The dither image is a two dimensional ordered dither pattern which has a rectangular array of dither samples repeating horizontally and vertically over the entire television scanning raster, or may be a three dimensional dither pattern which changes between successive frames of the televised moving picture. Color components as well as luminance components may be dithered before quantisation and this results in a pointillistically colored display wherein differently colored specks or patches are intermingled and give the subjective effect of intermediate colors or shades.